The present invention generally relates to television signal transmission systems and particularly concerns a television signal transmission system which facilitates the transmission of a digitally encoded high definition television (HDTV) signal on a television channel also being used in a nearby television service area for the transmission and reception of NTSC encoded television signals.
Much interest has recently been generated in so-called "digital" HDTV television systems. Such systems compress a wideband, generally up to about 37 MHz, video signal for transmission over a standard 6 MHz television channel. Various techniques have been proposed to effect transmission of the compressed signal; see for example, copending application Ser. No. 611,236, filed Nov. 9, 1990, now U.S. Pat. No. 5,087,975 and entitled VSB HDTV Transmission System With Reduced NTSC Co-Channel Interference. In contrast to the NTSC form of analog transmission, the HDTV signal is preferably transmitted in the form of a multilevel (i.e., two or more) video data signal, the latter form of transmission being truly digital in character since it is effected by employing a limited number of discrete transmission signal levels. Assuming that the interference threshold of the system has not been exceeded, such digital transmission systems afford the advantage that essentially perfect reproduction of the received signal is possible.
With regard to the matter of spectrum availability, it is anticipated that HDTV service will be provided over presently unused television channels to avoid obsolescence of existing NTSC services. It is further anticipated that at least in some television service areas HDTV transmission will be effected over channels also used in nearby television service areas for transmission and reception of NTSC signals. In the past, the former channels, sometimes referred to as "taboo" channels, have been unassigned to avoid cochannel interference between the nearby television service areas. However, due to the increased demand for spectrum utilization raised by the addition of HDTV service, it is anticipated, as previously mentioned, that previously unused taboo channels will, of necessity, be employed to at least partially effect this new television service In this regard, the issue of cochannel interference between nearby NTSC and HDTV transmissions must be addressed and, in particular, a solution to the problem of NTSC cochannel interference in a received digital HDTV transmission must be provided since excessive interference could largely negate the ability of the HDTV receiver to reproduce any meaningful information.
It is, therefore, a basic object of the present invention to provide an improved HDTV transmission system.
It is a more specific object of the invention to provide a system for reducing NTSC cochannel interference in a received digitally transmitted HDTV signal.